It is well known to manufacture paper by a process that comprises flocculating a cellulosic thin stock by the addition of polymeric retention aid and then draining the flocculated suspension through a moving screen (often referred to as a machine wire) and then forming a wet sheet, which is then dried. Some polymers tend to generate rather coarse flocs and although retention and drainage may be good unfortunately the formation and the rate of drying the resulting sheet can be impaired. It is often difficult to obtain the optimum balance between retention, drainage, drying and formation by adding a single polymeric retention aid and it is therefore common practise to add two or frequently three or more separate retention aids in sequence.
EP-A-366764 describes a process of making paper by drainage of a cellulosic suspension through a screen for a cellulosic sheet in which an aqueous solution of a polymeric retention aid is included in the cellulosic suspension before drainage. The polymeric retention aid is formed from water-soluble ethylenically unsaturated monomer and has an intrinsic viscosity of at least 12 dl/g and high solubility. The process is said to improve the formation of the paper without deterioration of the retention. It is also indicated that further retention additives may be included before the aforementioned polymeric retention aid.
In the manufacture of high quality paper such as fine paper or complex paper such as multiply packaging it is always essential that high retention of fibre and filler is achieved. As the industry strives to greater productivity paper products, such as fine paper or multiply packaging, are often now manufactured on very high speed paper machines, such as Gap Formers, and/or employing double wire dewatering processes. Although productivity is improved there is a greater tendency to create and introduce higher levels of fine materials. This in turn causes a significant reduction in retention performance.